Photonic integrated circuits hold the potential of creating low cost, compact, optical functions. The application fields in which they can be applied are very diverse: telecommunication and data communication applications, sensing, signal processing, and so on. These optical circuits comprise different optical elements, such as light sources, optical modulators, spatial switches, optical filters, photodetectors, etc., with the optical elements being interconnected by optical waveguides.
Optical waveguides are typically implemented as solid dielectric light conductors, which allow routing of light over the integrated optical circuit and to interconnect the various optical components integrated on the circuit. The optical waveguides also provide the interfacing between the optical fiber and the optical circuit, typically by physical abutment of the optical fiber to the waveguide. Due to the large difference in mode size between the optical fiber and the integrated optical waveguide, this typically leads to high coupling losses at the coupling interface.
Several solutions have been proposed for improving the coupling efficiency between an optical fiber and the optical waveguide circuit. In a first approach, the optical mode of the single mode optical fiber is transformed to a smaller spot-size by using a lensed optical fiber or a high numerical aperture fiber. Another approach is to use an integrated spot-size converter to expand the size of the integrated optical waveguide mode to match that of a single mode optical fiber.
As an alternative approach, vertically etched diffraction grating structures have been proposed. These structures allow direct physical abutment from the top or bottom side of an optical waveguide circuit with a standard single mode optical fiber, while the diffraction grating directs the light from the optical fiber into the optical waveguide circuit or from the optical waveguide into the optical fiber. However, when traditional gratings with a small coupling strength are used, long gratings are needed and the outcoupled beam is much larger than the fiber mode. As a result, an additional lens is needed to couple to a fiber or alternatively a curved grating can be used that focuses the light into a fiber. Another possibility is using strong index modulated gratings working in the strong coupling regime. Compact and efficient grating couplers with a rectangular tooth profile have been demonstrated in high index contrast material systems (e.g., SOI). For low-index contrast material systems (such as e.g. InP/InGaAsP heterostructures), it is not possible to design such a compact and efficient grating coupler.
However, e.g., for telecommunication applications, the material of interest is InP, allowing active functionality. InP heterostructures or InP/InGaAsP heterostructures can be easily integrated with active components, such as for example lasers or modulators. Typically, the index contrast for these material systems is modest and too low to make compact grating couplers. Only very long (typically hundreds of micrometers) and narrowband gratings have been demonstrated for such material systems.
To improve the coupling efficiency, slanted waveguide couplers or gratings have been proposed, for example as described in U.S. Pat. No. 5,657,407. In this approach, a grating coupler is provided with a row of parallelogramic teeth, whereby the side walls of the teeth form an angle different from 90 degrees with the plane of the waveguide corresponding to the guided wave propagation direction. The width to period ratio of the grating teeth may be variable along the guided wave propagation direction to shape the grating output beam profile. When using such slanted gratings, the radiation directionality and the radiation factor can be increased, resulting in an improved coupling efficiency. Gratings with a parallellogramic teeth profile can suppress second order reflection and show high radiation factors. Directionality of 90% was demonstrated in a low index-contrast material system, using parallellogramic gratings made by a modified RIE-process. However, the grating was 3 mm long. Moreover, for this type of grating, there are significant fabrication challenges due to the small periods and the steep slant angles required.
In “Compact slanted grating couplers,” Optics Express, Vol. 12, No. 15, 3313, July 2004, B. Wang et al. present a compact and efficient design for slanted grating couplers for coupling to polymeric waveguides, making use of a strong index modulated slanted grating. In this approach, the grating is etched in the cladding and filled with a higher index material. For a 20 μm long grating an input coupling efficiency of 80% is calculated, but experimental results are not reported.
In “Embedded slanted grating for vertical coupling between fibers and Silicon-on-Insulator planar waveguides,” IEEE Photonics Technology Letters, Vol. 17, No. 9, 1884, September 2005, Bin Wang et al. propose a slanted grating coupler in which the slanted grating is completely embedded in the waveguide core, with an overlying upper cladding that fills the grating grooves. Simulations show that up to 75.8% coupling efficiency can be obtained between a single-mode fiber and a 240 nm thick Silicon-on-Insulator planar waveguide. Experimental results are not shown. A typical groove size in the propagation direction is about 200 nm, which may be difficult to control during fabrication, and which may be difficult to fill with the cladding material.
In “Compact slanted grating couplers between optical fiber and InP-InGaAsP waveguides,” IEEE Photonics Technology Letters, Vol. 19, No. 6, 2007, F. Van Laere et al. describe a compact (10 μm long) slanted grating coupler for coupling between a single mode fiber and low-index-contrast InP-based waveguides (InGaAsP core and InP cladding). In a waveguide, narrow air slots are etched at an angle of 45 degrees with respect to the plane of the waveguide, completely through the core of the waveguide. Part of the light incident from the waveguide on the slots is reflected upwards at the interface with air. If the slots are sufficiently narrow, part of the light can tunnel through the slot and reach a second slot, where it is again partly reflected and partly transmitted. In this way, the waveguide mode is coupled out vertically in a distributed way, thereby matching the mode of a fiber positioned vertically above the grating.
An important design parameter for this type of grating couplers is the width (i.e., the size in the light propagation direction) of the air slots. When the slots are too wide, there is little transmission and almost all the light is coupled out by the first slot, resulting in a poor overlap with the fiber mode. When the slots are too narrow, most of the light tunnels through the slot and little light is reflected upwards. In a typical design, very narrow slots are required (narrower than 100 nm), and it is difficult to etch these slots sufficiently deep, the required depth being 1 μm or more. A compact grating coupler is obtained, but the coupling efficiency is relatively low. For a 10 μm long slanted grating the maximum calculated coupling efficiency is 59%, whereas an experimental coupling efficiency to fiber of 16% is reported for a non-optimized design.
In U.S. Pat. No. 7,184,625, a flared optical waveguide grating coupler is described for use in a high index contrast material system, the coupler comprising elongate scattering elements. In some embodiments, where features are required with dimensions smaller than the lithographic limit, e.g., for realizing a small scatter cross section, these elongate scattering elements are segmented. The segmented scattering elements are arranged over the waveguide core and their sidewalls are substantially perpendicular to the waveguide plane.